Gear differentials include a compound planetary gear set mounted within a differential housing. The planetary gear set interconnects a pair of output shafts for opposite directions of rotation with respect to the housing. The housing is rotatable by engine power about a common axis of rotation shared by the output shafts.
Ordinarily, the planetary gear set is arranged to permit the output shafts to rotate by equal amounts but in opposite directions with respect to the housing. Accordingly, the housing rotates about the common axis of the output shafts at the average speed of the two output shafts. Drive torque is distributed between the two relatively rotating output shafts in accordance with the efficiency of the planetary gear set.
Sun gear members of the planetary gear set, also referred to as "side gears", are coupled to inner ends of the output shafts. Planet gear members of the same set operatively connect the two side gears for rotation in opposite directions. Sometimes, the sun gear members and the planet gear members of the same set are mounted within the differential housing for rotation about parallel axis. Gear differentials with this type of gearing arrangement are referred to as "parallel-axis" gear differentials.
The planet gears of parallel-axis gear differentials are generally mounted in pairs within the housing. One portion of each planet gear meshes with one of the side gears, and another portion of each planet gear meshes with its paired planet gear. However, the planet gears are individually supported for rotation on shafts or within cylindrical pockets formed in the housing.
A commonly assigned U.S. Pat. No. 5,122,101 discloses an example of a parallel-axis gear differential in which the planet gears have main and transfer gear sections separated by a stem. The main gear section of a first planet gear member of a pair meshes with one of the side gears and with the transfer gear section of the second planet gear member of the pair. The main gear section of the second planet gear member meshes with the other side gear and with the transfer gear section of the first planet gear member. The two meshes between the paired planet gears straddle the two meshes between the planet and side gears.
Although the straddled planet gear meshes reduce tipping of the planet gears within their pockets, some misalignment between the gear meshes is still possible because of normal tolerancing and housing distortions under load. The misalignments can move contact between mating gear teeth to ends of the teeth. Higher load concentrations at the tooth ends can cause gear failures by pitting, scoring, or breakage. Tooth ends of misaligned gear members in contact with their end mounting surfaces can also cause significant housing wear.
The planet and side gear blanks are often made with small chamfers that bevel tooth tips at opposite ends of the gears. The chamfers, which are typically inclined at forty-five degrees from the gear ends, extend from the tooth tips to approximately the pitch line of the teeth. Although the known chamfers reduce wear between the gear ends and their mounting surfaces, the chamfers do not prevent edge contact between mating teeth. In fact, the known chamfers produce an angled tooth edge that is subject to chipping from edge contact. Any such chipping is considered a major failure because the chips can cause more significant damage elsewhere in the differential.